Various methods are known for welding plastic components, such as for example containment tanks and other types of special article.
One of the most widely used methods is welding with added material and is performed by means of an extruder composed of a front part, for plasticizing and ejecting plastic material to be added to the weld, and by a rear part for feeding and breaking up a continuous filament made of the additional material.
The front part, for plasticizing and ejection, generally comprises a plasticizing chamber, inside which the small pieces of plastic filament that originate from the rear part are melted and moved by means of a screw feeder toward an ejection nozzle.
Electric resistors for heating and melting the small pieces of filament are arranged around the plasticizing chamber.
The rear part generally comprises a cylindrical feed chamber, in which a worm screw is arranged coaxially.
A feed channel is provided between the thread of said worm screw and the wall of the cylindrical chamber.
The distance between the crest of the thread and the wall of the cylindrical chamber is shorter than the diameter of the plastic filament and accordingly the thread engages said filament.
The rotary motion of the worm screw (actuated by an electric motor) allows the filament to advance inside the feed channel until it reaches the plasticizing chamber.
At the end of the rear part, substantially at the end of the worm screw, there are generally elements for cutting the filament into many small pieces that will feed the plasticizing chamber.
The known extruders described here, although being known and used extensively on the market, are not free from drawbacks and aspects that can be improved.
A first drawback is that said cutting elements arranged at the end of the rear part do not provide uniform filament pieces, and this causes a less than optimum flow of additional welding material, since said material is not melted correctly.
A second drawback is linked to the fact that the rotating components, i.e., the screw feeder and the worm screw, must have a very precise axial tolerance, since any friction would damage the individual components, including the element that transmits rotary motion (which, if subjected to stresses caused by unexpected friction, is easily damaged or becomes noisy and tends to reduce its operating cycle); it should be noted, in this regard, that these extruders are subjected to very long and accordingly very demanding work cycles.
Another drawback is that the first part of the plasticizing chamber may overheat.
Overheating in this region can lead to an expansion of the rotating elements beyond the allowed tolerances and to consequent friction and malfunctions.